As is well known, in a "print-stopped" machine a character is printed only after the print mechanism is brought to rest at the specified print location. The escapement control system which governs the move to the next print location must move the carrier and print head assembly to a controlled stop in such a manner than any position undershoot or overshoot is within specifications. Any position undershoot or overshoot directly contributes to character registration error thus decreasing print quality. Additionally, the time required to move the print carrier and print head assembly to a controlled stop critically affects the overall throughput of the printer. As a result, the escapement control system must minimize position undershoot and overshoot without compromising move time and hence printer throughput.
The undershoot and overshoot problem becomes much more critical in a low cost printer when letter quality printing is desired. Such a low cost printer typically has a rail and belt drive system. In such a system, the coupling between the driving motor and the print head is not rigid with the result that there is insufficient mechanical damping which causes positional error between the motor and the print head. Undershoot and overshoot conditions will continually occur as the print head assembly attempts to get into phase with the driving motor. Another problem that can arise in such a low cost printer system is that the printer may be operating near the resonant frequency of the system. As is well known, this also contributes to system instability and hence compounds the undershoot and overshoot problems.
Two known methods of driving a print carrier assembly are first a constant velocity move, and second, a ramp velocity move. In a constant velocity move system, a velocity command of a constant amplitude over a fixed duration is provided to the print carrier assembly. In a ramped velocity move system, a ramped or triangular shaped velocity command is applied to the print carrier assembly. A major problem with both of these types of systems is that the print carrier response does not match the command velocity signal. As a result, undershoot and overshoot of the print carrier assembly will occur. This leads to print character misregistration and hence a decrease in print quality. With either of these two types of systems, some feedback sensing is required in order to provide letter quality printing in such a low cost printer.